High Lockout
F25 on Glow-worm Ultracom 35 Store

Your boiler has switched itself off because the internal exhaust gases have reached a temperature that is too high, often caused by a circulation problem or a lack of water.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Flue gas overheat fault

What causes this fault?

This fault occurs when the sensors detect that the exhaust gases leaving the boiler are getting much hotter than they should be, triggering an automatic safety shutdown. The most common reason is that heat isn't being moved away from the burner quickly enough, usually due to a lack of water pressure, a failing pump, or a blockage in the main heat exchanger.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

F22View →F23View →F24View →

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